1) Field of the Invention
The present invention relates to apparatuses and methods for preforming thermoplastic materials and, more specifically, to apparatuses and methods for bending thermoplastic sheets to form preforms for ducts.
2) Description of Related Art
Ducts provide transport passageways for a wide variety of applications. For example, tubular ducts are widely used for air flow in aircraft environmental control systems. Similarly, ducts provide passageways for transporting gases for heating and ventilation in other vehicles and in buildings. Water distribution systems, hydraulic systems, and other fluid networks also often use ducts for fluid transport. In addition, solid materials, for example, in particulate form can be delivered through ducts. Ducts for the foregoing and other applications can be formed of metals, plastics, ceramics, composites, and other materials.
One conventional aircraft environmental control system utilizes a network of ducts to provide air for heating, cooling, ventilation, filtering, humidity control, and/or pressure control of the cabin. In this conventional system, the ducts are formed of a composite material that includes a thermoset matrix that impregnates, and is reinforced by, a reinforcing material such as Kevlar®, registered trademark of E.I. du Pont de Nemours and Company. The thermoset matrix is typically formed of an epoxy or polyester resin, which hardens when it is subjected to heat and pressure. Ducts formed of this composite material are generally strong and lightweight, as required in many aircraft applications. However, the manufacturing process can be complicated, lengthy, and expensive, especially for ducts that include contours or features such as beads and bells. For example, in one conventional manufacturing process, ducts are formed by forming a disposable plaster mandrel, laying plies of fabric preimpregnated with the thermoset material on the mandrel, and consolidating and curing the plies to form the duct. The tools used to mold the plaster mandrel are specially sized and shaped for creating a duct of specific dimensions, so numerous such tools must be produced and maintained for manufacturing different ducts. The plaster mandrel is formed and destroyed during the manufacture of one duct, requiring time for curing and resulting in plaster that typically must be removed or destroyed as waste. Additionally, the preimpregnated plies change shape during curing and consolidation and, therefore, typically must be trimmed after curing to achieve the desired dimensions. The jigs required for trimming and for locating the proper positions for features such as holes and spuds are also typically used for only a duct of particular dimensions, so numerous jigs are required if different ducts are to be formed. Like the rotatable tools used for forming the mandrels, the jigs require time and expense for manufacture, storage, and maintenance. Additionally, ducts formed of conventional thermoset epoxies typically do not perform well in certain flammability, smoke, and toxicity tests, and the use of such materials can be unacceptable if performance requirements are strict. Further, features such as beads typically must be post-formed, or added after the formation of the duct, requiring additional manufacture time and labor.
Alternatively, ducts can also be formed of thermoplastic materials. A thermoplastic duct can be formed by forming a thermoplastic sheet of material, cutting the sheet to a size and configuration that corresponds to the desired shape of the duct, bending the sheet to the desired configuration of the duct, and joining longitudinal edges of the sheet to form a longitudinal joint or seam. For example, apparatuses and methods for forming thermoplastic ducts and consolidation joining of thermoplastic ducts are provided in U.S. application Ser. Nos. 10/216,110and 10/215,833, titled “Thermoplastic Laminate Duct” and “Consolidation Joining of Thermoplastic Laminate Ducts”, both of which are filed concurrently herewith and the contents of which are incorporated herein by reference. Such thermoplastic ducts can be formed by retaining the thermoplastic sheet in the bent configuration until the ends are joined, and then releasing the duct so that the resulting joint continues to restrain the duct in the bent configuration. However, stresses induced in the thermoplastic material during bending can cause the duct to deform or distort from the desired configuration after joining, e.g., when released from the joining apparatus.
Thus, there exists a need for improved apparatuses and methods of preforming ducts, i.e., providing a preform configured to correspond generally to the desired configuration of the duct in a substantially unstressed condition. The method should not require the laying of individual plies on a disposable plaster mandrel. Preferably, the method should be compatible with thermoplastic ducts, including reinforced thermoplastic ducts formed from flat sheets, which provide high strength-to-weight ratios and meet strict flammability, smoke, and toxicity standards.